Method and apparatus for tracking invalid signals through a digital system

ABSTRACT

A tracking apparatus identifies distorted output signals from a digital system caused by at least one distorted signal input to the digital system. A first signal is input to the tracking apparatus when a distorted signal is input to the digital system. A second signal is input to the tracking apparatus when a non-distorted signal is input to the digital system. The tracking apparatus models a delay and widening characteristics of the digital system and operates in parallel with the digital system.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority benefit of U.S. ProvisionalApplication No. 60/259,121, filed on Dec. 29, 2000, which isincorporated herein by reference.

DESCRIPTION OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates in general to a method andapparatus for tracking distorted signal through a digital system. Moreparticularly, the present invention relates to a method and apparatusfor identifying distorted output signals from a digital filter caused bya distorted input signal to the digital filter.

[0004] 2. Background of the Invention

[0005] A distorted signal in a digital system may result because adigital component in the digital system operated beyond its dynamicrange due to a multiplication or summation operation in the digitalsystem, for example. Analog-to-digital conversion and digital filteringare common signal processing algorithms performed in a digital systemthat cause distorted signals in the digital system. In addition todynamic range overflow, distorted signals in a digital system may resultfrom distorted input signals to the digital system. For example, indigital filters, a single distorted input signal may create a stream ofdistorted output signals. This occurs because of the wideningcharacteristics of a digital filter due to non-zero filter coefficientsof the digital filter. In addition, the effect of a distorted inputsignal on output signals of a digital filter may be delayed due to thepipelined implementation of the digital filter. That is, if a distortedinput signal is input to the digital filter at time t₁, then thedistorted input signal may cause a distorted output signal from thedigital filter at time t₂, where t₂>t₁.

[0006] Distorted signals may reduce the accuracy of the digital systemand generate erroneous data. For example, if a distorted signal in adigital system is used to compute filter weights for a digital filter,the accuracy of the digital system may be reduced. Therefore, it isdesirable to track the distorted signals so that their deleteriouseffects may be reduced by, for example, excluding the distorted signal.

[0007] However, conventional tracking devices only identify distortedsignals that result from internal overflows and do not identifydistorted signals that result from distorted input signals. Accordingly,there is a need for a tracking device for identifying distorted signalsresulting from distorted input signals.

SUMMARY OF THE INVENTION

[0008] In accordance with one embodiment of the invention, there isprovided a method of identifying distorted output signals from a digitalsystem caused by at least one distorted input signal to the digitalsystem. The method comprises modeling a delay and wideningcharacteristics of the digital system to provide a tracking circuit. Themethod further comprises providing an input signal to the trackingcircuit wherein the input signal has a first value when a distortedsignal is input to the digital system and has a second value when anon-distorted signal is input to the digital system. The trackingcircuit is operated in parallel with the digital system and providesoutput signal having a first value to indicate that a correspondingoutput signal from the digital system is distorted or a second value toindicate that a corresponding output signal from the digital system isnot distorted.

[0009] Additional advantages of the invention will be set forth in partin the description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention will be realized and attained by meansof the elements and combinations particularly pointed out in theappended claims.

[0010] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

[0011] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate several embodimentsof the invention and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 illustrates an exemplary filter that models a digitalfilter.

[0013]FIG. 2 illustrates a digital filter and tracking circuit operatingin parallel.

[0014]FIG. 3 illustrates an exemplary tracking circuit according to theteachings of the present invention.

[0015]FIG. 4 illustrates an exemplary tracking circuit according to theteachings of the present invention.

[0016]FIG. 5 illustrates an exemplary simplified tracking circuitaccording to the teachings of the present invention.

DESCRIPTION OF THE EMBODIMENTS

[0017] Reference will now be made in detail to the present exemplaryembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

[0018] As discussed above, it is desirable to track distorted signalsthrough a digital system. A single distorted input signal to a digitalsystem may create a plurality of distorted output signals. Further asdiscussed above, these distorted output signals may be delayed from thetime the distorted input signal is input to the digital system. Todetermine which output signals of a digital system are distorted, atracking circuit operating in parallel with the digital system isprovided. Each input signal to the tracking circuit corresponds to aninput signal to the digital system. Each output signals from thetracking circuit corresponds to an output signal from the digitalsystem.

[0019]FIG. 1 illustrates an exemplary tracking circuit 110 operating inparallel with a digital filter 120. For each input signal 125 to thedigital filter 120, a corresponding high or low signal 115 is input tothe tracking circuit 110 to indicate whether the input signal 125 to thedigital filter 120 is distorted. For example, if the input signal 125 tothe digital filter 120 is distorted, a high signal (i.e., an input flag)is input to the tracking circuit 110. Otherwise, a low signal is inputto the tracking circuit 110. For each output signal 130 from the digitalfilter 120, a corresponding high or low signal 135 is output from thetracking circuit 110 to indicate whether the output signal 130 from thedigital filter 120 is distorted. For example, if the output signal 135from the tracking circuit 110 is high, then the corresponding outputsignal 130 from the digital filter 120 is considered distorted. If theoutput signal 135 from the tracking circuit 110 is low, then thecorresponding output signal 130 from the digital filter 110 isconsidered valid.

[0020] For the tracking circuit 110 to operate in parallel with thedigital filter 120, the tracking circuit 110 may model the delay andwidening characteristics of the digital filter 120. FIG. 2 illustrates afilter model 200 that models the operation of a (n+1) coefficientdigital filter typically found in a digital processing system. Morespecifically, the filter model 200 models the delay and wideningcharacteristics of the digital filter. The filter model 200 comprisesdelay means 202 and 204, multipliers 206, and adders 208.

[0021] Delay means 202 delays an input signal for a period of timeequivalent to an initial latency period in the digital filter. Theinitial latency period is the time it takes for the input signal toeffect an output signal of the digital filter. Delay means 204 furtherdelays the input signal to form delayed input signals for furtherprocessing. The filter model 200 comprises (n+1) multipliers 206 formultiplying the delayed input signals by (n+1) filter coefficients W₀,W₁, W₂, . . . W_(n−1), W_(n), respectively, to form weighted inputsignals. Adders 208 sum the delayed and weighted input signals to forman output signal.

[0022] As discussed above, one or more distorted input signals may causedistortion in a plurality of output signals because of the wideningcharacteristics of the digital filter due to the non-zero filtercoefficients. The filter model 200 models the widening of the distortedinput signal using delay means 204, multipliers 206, and adders 208.Furthermore, the distorted output signals will be delayed from the timethe distorted input signal is input to the digital filter. The filtermodel 200 models the delay of the distorted output signals using thedelay means 102.

[0023] Similar to filter model 200, the tracking circuit 300 of FIG. 3models the delay and widening characteristics of a digital filter.However, the tracking circuit 300 is a simplified version of filtermodel 200. Tracking circuit 300 comprises delay means 302 and 304 andadders 308. Because a single bit may be used to indicate whether asignal is distorted, the adders 308 may be implemented using two-inputOR gates 408, for example, as shown in FIG. 4. Furthermore, because thetracking circuit 300 tracks distorted signals and does not performdigital filtering similar to filter model 200, multipliers are notneeded. For each non-zero filter coefficient in the digital filter, thetracking circuit 300 uses a multiplier of one. For each coefficientequal to zero in the digital filter, the tracking circuit 300 grounds aninput to a corresponding adder 308. For example, as shown in FIG. 4, foreach filter coefficient equal to zero in the digital filter, thetracking circuit 300 grounds an input to a corresponding OR gate.

[0024] Tracking circuit 300 of FIG. 3 may be further simplified if thenon-zero filter coefficients of the digital filter are contiguous. Thenon-zero filter coefficients may be contiguous if there are no filtercoefficients equal to zero between the first non-zero filter coefficientand the last non-zero filter coefficient. In the case of contiguousnon-zero filter coefficients, tracking circuit 500 of FIG. 5 may be usedto track distorted input signals. The tracking circuit 500 models thedelay of the digital filter using delay means 502. Delay means 502delays an input signal to the tracking circuit 500 for a time equivalentto an initial latency of the digital filter.

[0025] If a distorted signal is input to a digital filter withcontiguous non-zero filter coefficients, the length of the chain ofdistorted output signals from the digital filter may be equal to thenumber of non-zero filter coefficients of the digital filter. Thiswidening characteristic of the digital filter may be modeled using apresetable down counter 504 and decision unit 514, for example. Thepresetable down counter 504 and decision unit 514 model the wideningcharacteristics of the digital filter by creating a chain of high outputsignals equal in length to the number of non-zero filter coefficients ofthe digital filter to indicate that the corresponding chain of outputsignals of the digital filter are distorted.

[0026] The tracking circuit 500 may operate as follows. When a distortedsignal is input to the digital filter, a high signal is input to thetracking circuit 500 causing the down counter 504, once the high signalreaches the preset input 506, to preset and begin counting down from apreset value. The preset value may be equal to the number of contiguousnon-zero filter coefficients, z. The preset value is set using thepresent value input 508 of the down counter 504. The down counter 504continues to count down until either the down counter 504 reaches zeroor until another high signal is input to the down counter 504 causing itto preset to the preset value. For each output signal from the downcounter 504, decision unit 514 determines whether the count vale isnon-zero. If the count value 510 is non-zero, then decision unit 514outputs a high signal, indicating the corresponding output signal fromthe digital filter is distorted. If the count value 510 is zero,decision unit 514 outputs a low signal, indicating the correspondingoutput signal from the digital filter is valid. When the down counter504 reaches zero, it remains at zero until a subsequent high signal isinput to the down counter 504. It will be obvious to one of ordinaryskills in the art the various means of implementing decision unit 514.For example, decision unit 514 may be implemented using one or morelogic elements.

[0027] The tracking circuit of FIG. 5 may be used if the non-zero filtercoefficients of the digital filter are noncontiguous. In this case, thepreset value of the down counter 504 is equal to the number of weightsfrom the first non-zero weight to the last non-zero weight. However, thetracking circuit may indicate that some output signals of the digitalfilter are distorted when they are not.

[0028] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method of identifying distorted output signalsfrom a digital system caused by at least one distorted input signal tothe digital system, the method comprising: modeling a delay and wideningcharacteristics of the digital system to provide a tracking circuit;providing an input signal to the tracking circuit wherein the inputsignal has a first value when a distorted signal is input to the digitalsystem and has a second value when a non-distorted signal is input tothe digital system; operating the tracking circuit in parallel with thedigital system; and providing an output signal from the tracking circuitwherein the output signal has a first value to indicate that acorresponding output signal from the digital system is distorted and hasa second value to indicate that a corresponding output signal from thedigital system is not distorted.
 2. The method of claim 1, wherein thedigital system is a digital filter.
 3. The method of claim 2, whereinmodeling a delay and widening characteristics of the digital systemcomprises providing a digital filter model that models the delay and thewidening characteristics of the digital filter.
 4. A method ofidentifying distorted signals output from a digital filter caused by atone distorted signal input to the digital filter, the method comprising:delaying an input signal for a predetermined amount of time when adistorted signal is input to the digital filter wherein the input signalhas a first value when a distorted signal is input to the digital systemand has a second value when a non-distorted signal is input to thedigital system; providing the delayed first input signal to a counter;setting a counter to N and decrementing the counter by one until eitherthe counter reaches zero or a subsequent signal is received by thecounter that equals the first value; resetting the counter to N if thesubsequent signal equals the first value; and outputting an outputsignal wherein the output signal has a first value if a counter outputis non-zero to indicate a corresponding output signal from the digitalfilter is distorted and has a second value if the counter output is zeroto indicate a corresponding output signal from the digital filter is notdistorted, wherein N equals a number of filter coefficients of thedigital filter from a first non-zero filter coefficient to a lastnon-zero filter coefficient.
 5. A method of identifying distortedsignals output from a digital system caused by at least one distortedsignal input to the digital system, the method comprising: providing atracking circuit which models the delay and the widening characteristicsof the digital system; and operating the tracking circuit in parallelwith the digital system.
 6. The method of claim 5, wherein the digitalsystem is a digital filter.
 7. The method of claim 6, wherein thetracking circuit is a digital filter model that models the delay and thewidening characteristics of the digital filter.
 8. An apparatus foridentifying distorted signals output from a digital filter caused by adistorted signal input to the digital filter, the apparatus comprising:means for modeling the delay of the digital filter; and means formodeling the widening characteristics of the digital filter.
 9. Anapparatus for identifying distorted output signals from a digital filtercaused by at least one distorted signal input to the digital filter,wherein a first signal is input to said apparatus having a first valuewhen a distorted signal is input to the digital filter or a second valuewhen a non-distorted signal is input to the digital filter, the circuitcomprising: a delay circuit to receive the first signal and to delay thefirst signal for a predetermined time forming a delayed first signal; acounter to receive the delayed first signal, wherein the counter isconfigured to count down from N until either the counter reaches zero ora subsequent signal is received by the counter that equals the firstvalue and configured to reset to N if the subsequent signal equals thefirst value; and a decision unit to determine whether an output from thecounter is non-zero, wherein N equals a number of filter coefficients ofthe digital filter from a first non-zero filter coefficient to a lastnon-zero filter coefficient.